Seabirds are often found nesting in large numbers on islands near coral reefs. They forage for food out in the open ocean and come back to nest and breed in island colonies. Oahu, the capital-city island of Hawaii with roughly 1 million residents is surrounded by several small islets that are mostly uninhabited by humans. It is on these relatively secluded islets that the majority of the seabird population of Oahu makes their home (Fig. 1).

Coral reefs and seabirds are both threatened marine communities. Their coexistence in close proximity to one another may help the coral reef community thrive by adding concentrated oceanic nutrients to the system. Seabirds find food far out at sea and the nutrients they consume become concentrated in their guano (a.k.a. poop). When the guano washes back into the ocean from their island nesting grounds, these nutrients may become available to the marine environment of the nearby coral reefs. In this way, seabirds connect offshore and coastal marine environments and turn the food chain into something of a loop, connecting the top directly back to the bottom.

Working in some of the isolated islets off Oahu, Honig and Mahoney designed an experiment to determine whether seabird-derived nutrients really do end up in coral reef waters.

A natural experiment

During the rainy season, when seabird guano on islands could most easily wash into the ocean, Honig and Mahoney studied four islets and their neighboring reefs (see map, Fig. 2). Moku Nui, the southeastern-most island has a very large seabird population while fewer seabirds live on the other three islets. All four islets are uninhabited by humans and have coral reefs nearby. Mokoli’I (in the northwest) is arguably the most influenced by human activities since it is nearest any freshwater runoff, but all of the islets are relatively isolated from human activity.

Figure 2. a) Coconut Island in Kanehoe Bay, Hawaii is home to the Hawaiian Institute of Marine Biology which served as the research base for this work (Image from https://www.hawaii.edu/himb/). b) Map of study sites. (Figure 1 in the paper.)

Results

The island with the large seabird population had more dissolved phosphate in the waters nearby than any of the other islands. Phosphate is an essential nutrient that plants need to grow; more phosphate in the water means more resources are available to plants in the coral reef ecosystem. Just because they’re available, though, doesn’t mean that the extra nutrients are being used. To be sure, the researchers also sampled the macroalgae Halimeda (Fig. 3) and found that the seabird-derived nutrient signal also showed up there. Fish and invertebrates feed upon these macroalgae, so a boost in production at the plant level can affect higher and higher trophic levels. Not only are nutrient concentrations higher near the seabird colonies, but their guano could be fueling production at all levels of the coral reef ecosystem.

The link between seabirds and nutrient enrichment becomes complicated in human-influenced areas, like Oahu, where nutrient enrichment can also occur as a result of unnatural causes like runoff from sewage and fertilizers. Too much nutrient loading can often have harmful effects on highly diverse coral reef ecosystems by favoring fast-growing algal species, which tend to support low-diversity ecosystems.

Sewage and fertilizer runoff exist on Oahu and macroalgae growing near those sources show signals that indicate it utilized those unnatural nutrient sources, so how do we know the excess nutrients in the waters off Moku Nui came from the birds? First of all, the islets studied here are relatively far from any direct sources of anthropogenic nutrient input. More importantly, of the four islets studied, only the one with a large seabird population had higher nutrient concentration in the water and the algae. Since all the islets lie in similar proximity to human activities, we can safely assume that the excess nutrients came from the seabirds.

A complicated story for conservation

Seabirds are endangered and conservation efforts are underway to help them recover. But corals are also endangered. Interactions are between the two groups are complicated, and still not well understood. It is possible that increasing the number of seabirds on the island could lead to over-loading of nutrients on reefs that are already under stress due to their proximity to human activity. It’s also possible that these excess nutrients could support a larger fish population that would feed on the excess algae. Conservation efforts, therefore, should make sure to take the whole system into consideration. For example, combing efforts to rebuild seabird populations with stricter fishing regulations may bring the ecosystem into balance. Seabirds and coral reefs coexist naturally in many locations and, with careful planning, both communities should be able to thrive in the Hawaiian Islands.

I’m interested in how physical processes occurring in different parts of the ocean affect local ecosystems and climate. For my PhD research at Rutgers University (New Brunswick, NJ), I am studying the circulation and pathways of heat transport in the waters of the West Antarctic Peninsula continental shelf, one of the fastest warming regions of the planet. When I’m not thinking about the ocean, I do a lot of swim-bike-running and compete very uncompetitively on the Rutgers Triathlon team.

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Discussion

One Response to “Seabird poop and coral reefs”

1. The article is called: Seabird poop and coral reefs by Nicole Couto.

2. Scientists believe that off the islands of Oahu, seabirds and coral reefs are benefiting. Surprisingly enough, the guano, or seabird poop, may be positively affecting the marine and coral life in several coral reefs. Since seabirds and coral reefs are both endangered, they exist close to one another and benefit each other in unique ways. When seabirds go out into open ocean for food, their nutrient-rich guano goes into the sea, and when it washes into shallower waters where coral reefs reside, and becomes available to marine communities. An experiment was conducted to see whether or not seabird guano affects marine life by studing four, un-inhabited by human islets, and the one with the largest concentration of seabirds had the most dissolved phosphate in nearby waters than any of the other islets. Since phosphate is a nutrient plants need to grow, the marine life benefited greatly from the seabirds’ contributions. Researchers also studied the Halimeda, a plant, and found traces of the phosphate and guano within it as well. Since marine life feed off these plants, the whole ecosystem was given an upgrade on the nutrients they received. However, in islands populated by humans, such as Oahu, the sewage and fertilizer runoff can negatively affect the waters that should only beaffected by seabird guano. If the runoff contains too many nutrients, it could only favor a certain type of algae which can cause a low diversity marine life variation.

3. Comment: I think that the fact that something as strange as seabird guano can positively affect marine life, and that humans who inhabit islands with coral and seabird populations should try to stop all runoff and fertilizers, because they can drastically alter the ecosystem. I believe that the author explained, very well, the facts of the interaction between seabirds and coral reefs and that this relationship is not yet understood. This article helped me learn that natural interactions such as that between the seabirds and the coral reefs can be positive, until mankind comes into the picture and can cause a negative affect to take place by depositing man-made products in the water. A question I have for the author is: Do different types of seabirds eat different types of food, which results in different nutrient-based guano, which affect the rate that a marine ecosystem improves or doesn’t improve?